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Math Help - finding radius of a circle - please help!

  1. #1
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    finding radius of a circle - please help!

    could anyone quickly help me find this radius...i am totally stuck here and have to know this before i can move forward.

    Find the standard form of the equation -

    center is (8, 3) and passes thru (-6, -1)


    so far i have figured out (x-8)^2 + (y-3)^2 = r^2

    the radius is what i cannot understand what to figure out...
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  2. #2
    Eater of Worlds
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    Use the distance formula:

    d=\sqrt{(x-x_{1})^{2}+(y-y_{1})^{2}}
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  3. #3
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    Yes, you're on the right track. Now all you have to do is... Substitute  x = -6 and  y = -1 into the same equation to get r.

    I hope that helps.

    ILoveMaths07.
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  4. #4
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    Quote Originally Posted by shipwreck View Post
    could anyone quickly help me find this radius...i am totally stuck here and have to know this before i can move forward.

    Find the standard form of the equation -

    center is (8, 3) and passes thru (-6, -1)


    so far i have figured out (x-8)^2 + (y-3)^2 = r^2

    the radius is what i cannot understand what to figure out...
    The radius of a circle is by definition, the distance from the center to any point on the circle. What is the distance from (8, 3) to (-6, -1)?

    Another perfectly good way to do this is to say that, since (-6,-1) is ON the circle, it must satisfy that equation. If x=-6 and y= -1, what does your equation give you?
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